The invention relates to a method for applying retention means onto casting patterns of dental prosthetic metal constructions.
Normally teeth missing to a substantial degree are replaced by a metal construction in the form of a crown or bridge. For cosmetic reasons the portion of the metal construction visible in the mouth is covered at least partially by a composition having the color of natural teeth. Besides ceramic compositions, special synthetic materials are used for this purpose. In order to firmly anchor the synthetic material to the metal construction, the surface of the metal construction is provided with so-called retention means. Normally these retention means are small beads (of 0.2 to 0.8 mm diameter, for example) but also, specially fitted pieces of honeycomb and grid structures are employed.
Such retention means, on the one hand, offer improved adhesion owing to the enlarged surface area, while at the same time they prevent detachment of the synthetic resin facing under tensile, shear, or compressive stress by virtue of their cross section which tapers towards the metal construction.
The metal constructions are produced according to the lost wax casting technique. For this purpose the configuration of the prosthetic metal construction is modelled in wax on a plaster model of the dentition. This so-called casting pattern is then embedded in a refractory mold, the wax is removed by melting or burning, and the liquid metal is introduced in a manner known per se.
It has proved impractical to model the rentention means in wax, because undercuts pose a modelling problem, and this procedure is time-consuming and awkward.
It has been known to first apply an adhesive to the wax surface and then to place, on the adhesive, synthetic resin beads, or grid or honeycomb structures of wax or plastic, which are removable completely thereafter e.g. by burning or melting. The casting pattern can then be embedded as usual, the wax and the retention bodies can be removed, and the liquid metal can be poured in.
This method leaves relatively little time to apply the retention means, since the adhesives dry after a short time and are no longer sticky. Therefore, careful and proper distribution of the retention means over the surface of the casting pattern is frequently not possible.
If the adhesive layer is too thick--due to or promoted by a dried-up adhesive mix--the retention bodies can partially sink into the adhesive up to more than their maximum diameter. Hence, the required extent of surface enlargement is not attained. However, it is even more detrimental that above the casting surface there extend only few or no parts of the retention means whose cross section tapers toward the surface of the casting pattern. It will be easily understood that these shortcomings greatly reduce the adhesion of the plastic facing to the metal surfaces.
In addition, for cosmetic reasons it is normally necessary to cover the metal construction with a so-called opacifier before applying the synthetic resin facing material, i.e. to cover the metal color by an opaque film having a color similar to that of teeth. Frequently these opacifier compositions have less strength than the synthetic resin facing material. Also, after application of the opacifier material in a thickness covering the substrate, undercuts at the retention means must still be available which can be filled by the synthetic facing material. For this reason, it is desirable that, as far as possible, the retention means be anchored to the metal surface only punctually, leaving a maximum downwardly tapering portion of the retention means which is available for adhesion of opacifier compositions and synthetic facing material.